perf bpf: Give precedence to bpf header dir

perf bpf: Give precedence to bpf header dir

I need to check the need for $KERNEL_INC_OPTIONS when building eBPF
restricted C programs, for now just give precedence to
$PERF_BPF_INC_OPTIONS so that we can get a linux/socket.h usable
in eBPF programs.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: https://lkml.kernel.org/n/tip-5z7qw529sdebrn9y1xxqw9hf@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>